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The primaeval lunar magnetic field: Its relevance to the interpretation of planetary fields
Adv. Space Res. Vol. 12, No. 8, pp. (8)165—(8)186, 1992 Printed in Great Britain. All rights reserved.
0273—1177/92 $15.00
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1992 COSPAR
THE PRIMAEVAL LUNAR MAGNETIC FIELD: ITS RELEVANCE TO THE INTERPRETATION OF PLANETARY FIELDS S. K. Runcom Physics Department, Imperial College, London, U.K. (and University ofAlaska, Fairbanks, AL, U.S.A.)
ABSTRACT: The Moon today has no general magnetic field but the remanant magnetization of rocks returned and crustal magnetic fields show that it had one once. Palaeointensity laboratory measurements show the field approached 0.1 mT (1G) 3.9 Ga ago, decreasing exponentially to about 0.002 mT 3.2 Ga ago: evidence for a small iron core dynamo. Palaeomagnetic poles, determined from satellite magnetic surveys, show the Moon to have reorientated several times with respect to its axis of rotation and to have had a primaeval satellite system, the decay of their orbits producing the multi-ring impact basins. I conclude that planetary field strengths vary with evolutionary factors, such as the energy sources for convection; thus doubt is thrown on “scaling laws”. Vigorous dynamos do shut down as the Moon shows. Has the same occurred in Mars and Venus? Planetary dynamos are possible on small length scales: thus the off center dipoles of Uranus and Neptune may be local dynamos in their water-ammonia fluid spherical shells. While the mean geomagnetic and lunar fields are central dipoles, explained by the dominance of the Coriolis force in their cores, perhaps in this case the local dipoles align along the angular rotation component perpendicular to the shell. Are the Uranus and Neptune dipole inclinations so explained? Introduction Any account of what we know about the internal magnetic fields of the planets, especially in relation to the dynamo theory of their (8)165
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generation, needs to include the ancient magnetic field of the Moon. This has been one of the most surprising discoveries from the Apollo rocks. It is worth remembering that only thirty years ago we knew nothing about the magnetic fields of any of the planets other than the Earth, and in 1946 there was no theory of the origin of the Earth’s magnetic field. The dynamo theory was first proposed by Elsasser (1946 and 1947), but Blackett (1947) speculated immediately following the first discovery of a magnetic field of a star (78 Virginis) that perhaps magnetic fields of cosmic bodies might be a new property of rotating matter. At the time Einstein was much concerned with the possibility of a unified theory of electromagnetism and gravitation: so Blackett’s proposal was taken seriously as a possible observational clue. Chapman, who had written little about this question in his treatise (Chapman and Bartels 1940), was looked to for an opinion. He proposed that the theory’s prediction, that the Moon’s surface field was 100 ‘y, should be tested (Chapman 1947). He pointed out that the development of rockets and of airborne magnetometers and the use of radio for communication of information had taken vast strides during World War II and had made such an experiment possible. So he proposed the experiment, which seemed to everyone at the time science fiction, that was done by the Soviet Union two years after Sputnik finding, to an accuracy of about 100 ‘y, that the Moon had no magnetic field. The development of studies of the Moon, in the years around 1960, focused on the origin of the craters. As a result of the work of Baldwin (1949,1963) and Urey (1952) it was established that almost all the surface features of the Moon were of external origin, particularly that the craters were the result of meteorite impacts. It therefore became the established orthodoxy that the Moon was a sample of undifferentiated solar system material resulting from early cold accretion from the nebula of gas and dust. This was urged as a very strong argument for going to the Moon. On this hypothesis an iron core would not have formed and, as the core dynamo theory of Elsasser and Bullard was providing a satisfactory explanation of the main geomagnetic field, the negative result of the Soviet spacecraft experiment, showing that the Moon had no magnetic field, fitted perfectly: a liquid iron core was necessary for generating a planetary field. Consequently it was one of the most unexpected discoveries of the Apollo project that the lunar rocks
The Primaeval Lunar Magnetic Field
possessed remanent magnetization. The development of research on the palaeomagnetism of terrestrial rocks had provided insight into the various ways in which this magnetization is acquired. As a result of experiments done in a number of laboratories it became clear that the remanent magnetization of the Apollo rocks had not been picked up adventitiously since collection but had been acquired in some early process on the Moon; either at the formation of the rock or during its later exposure to meteorite bombardment or to the solar wind. The question at issue was the origin of the magnetic field in which the lavas in the mare basins and the breccia or fragments of highland rock had cooled and acquired magnetization. Because of the general opinion that the moon was a cold undifferentiated body it seemed to most lunar scientists that the magnetic field, which had magnetized the Apollo rocks, could not be an early general lunar magnetic field but must have been generated in a way unfamiliar to us on Earth. The lunar core dynamo However for reasons that I will explain later, we put forward the hypothesis that the Moon had a small iron core which could have been molten in its very early history: therefore a dynamo process was a reasonable hypothesis (Runcorn et al 1970). The important conclusion followed that the remanent magnetization of the lunar rocks was a record of the changes in this ancient lunar magnetic field. A serious obstacle to these studies was that the Apollo rocks were not oriented because they were pieces of the underlying strata broken off by impact and lying on the regolith. It is possible that methods could be developed for finding the ancient horizontal in the lava samples by magnetic and mineralogical anisotropy determinations, assuming that the flow aligns some of the grains. However at the present time there is no means for establishing the direction of the ancient magnetic field from the returned Apollo rocks. Thus the laboratories concerned with this work turned their attention to paleointensity determinations i.e. the strength of the ambient field responsible for its magnetization. There are three different methods. The Thellier-Thellier method using thermo-
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remanent magnetization (T.R.M. ), that based on anhysteritic remanent magnetization (A.R.M.) and the method that compares the natural remanent magnetization (N.R.M.) intensity with that produced in a very strong magnetic field, called isothermal remanent magnetization (l.R.M.). What these methods do is to compare the N.R.M. with what the rocks can pick up from a small laboratory field: in the case of the Thellier-Thellier method the N.R.M. is compared with that produced by cooling the rock in the laboratory from above the Curie point of its magnetic constituent which are small grains of iron. Stephenson et al (1975) discussed the change in the strength of the ambient field responsible for the magnetization of Apollo samples of different age. They found that the very earliest samples of 3.9 G yr had been magnetized by a field of almost 1 gauss (1 0~tesla) rather larger than the Earth’s polar field today. This was a result that seemed unlikely particularly to those who thought that the strength of a planetary magnetic field was related to the size of the core and to the rotation rate. However I think that it is more reasonable that the strength of the field produced by a dynamo process is dependent on the amount of energy driving the convection: even a small core can produce a high field if it is driven vigorously enough. The exponential decay of the field found by Collinson et al (1977) can be naturally interpreted as being due to the gradual diminution of the heat sources available for driving convection in the lunar core. These experiments are not easy and more results are required. The present state of knowledge has been improved by the approximate palaeointensity estimates by Cisowski et al (1983) by means of standardization of the N.R.M. which, though inherently scattered, support the finding that the field was very strong 3.9 G yr ago and declined exponentially to about .02 of this value 3.2 G yr ago. It is significant that the magnetic field was present during the period of extensive volcanism, that flooded the basins; heat was being transported in the mantle to the surface favoring core convection and dynamo action. Volcanism and field generation ceased somewhat later. Some evidence has emerged, as yet only suggestive, that the field rose prior to the maximum: if this is correct it might be a most important key to primeval lunar processes. The main conclusion of the paleointensity study is that the field in which the rocks cooled was a function of age. This is more easily explained by the dynamo process than by the various
The Primaeval Lunar Magnetic Field
other alternative suggestions as to the magnetizating agent that have been made i.e. transient magnetic fields locally generated, particularly as a result of meteorite impacts. Palaeodirections and palaeopoles Although the returned Apollo samples do not enable the direction of the ancient lunar field to be found, it has been possible to obtain palaeomagnetic directions from the magnetic anomalies observed by the 3-component magnetometer on the Apollo 15 and 16 subsatellites (Coleman and Russell 1977). It was on the far side of the moon that the magnetic anomalies were first charted: anomalies in the vertical and horizontal field intensities which arise from strata at the lunar surface. The coverage of the magnetic anomalies will remain small until the lunar polar orbiter mission is carried out. However I will show that the present small coverage is not as big a handicap as might be supposed. Coleman and Russell first fitted these very small anomalies by dipoles near the surface of the moon and they found that, to obtain an optimum fit, the dipole had to be placed about 50 km below the surface. At this depth the temperatures are above the Curie point so that rocks could not be magnetized. The correct interpretation of this important result depends on an interesting theorem: a uniformally magnetized thin disc (on the surface of the moon) produces a field exactly that of a dipole placed below the surface at the lowest point of a sphere centered at the height of the observing satellite and circumscribing the disc. So it was inferred that what was being measured by the subsatellite magnetic survey is a mean magnetization of very extensive strata, hundreds of km in lateral extent, on the lunar surface. Thus Coleman and Russell (1977), Hood et al (1978, 1979) and Hood (1981) determined directions of magnetization of different rock strata on the surface of the moon. For a core dynamo operating in the early moon the Coriolis force would have been dominant in its magnetohydrodynamic equation, as in the Earth’s core. Therefore there is a strong reason to suppose that the mean lunar magnetic field must have been a dipole aligned along the axis of rotation. For the Earth the paleomagnetism of the late Tertiary has shown that, averaging out the secular variation of the Earth’s field, an axial dipole field is found. So it is possible from lunar paleomagnetic
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data to calculate the ancient pole positions. Fig. 1 is a stereographic projection of the east and west hemispheres of the Moon with the north magnetic poles, calculated from the magnetic anomalies on the far side of the Moon, plotted. The stars in these diagrams are vector means of the two opposed groups. They are about 180°apart and lie near to the present equator of the moon. This antipodal arrangement of north poles suggests that the lunar core dynamo reversed just as the Earth’s field reverses, and that the two preferred fields are on average 180°apart as in the case of the Earth. Thus the stars define the magnetic axis of the moon and also its axis of rotation at that time. From the preponderance of Pre-Nectarian formations in this central far side of the Moon, I conclude that these pole positions are Pre-Nectarian. I have hitherto estimated this time at 4.2 G yr. I conclude that at this epoch the moon was rotating about an axis 90°to its present axis of rotation relative to its surface, although fixed in space apart from precession. As the paleoequators are at right angles to the present equator, the multi-ring basins of that age, Pre Nectarian, were formed by great impacts in low latitude as seen in Fig. 3. The absence of these very ancient impact basins on the present front side of the moon is due to the very extensive flooding of lava between 3.9 and 3.2 G yr ago covering the very faint traces of these very ancient impacts identifiable on the far side. The numbered magnetic poles were derived from anomalies modelled by Hood et al (1978): the lettered poles were later calculated from paleodirections from far side anomalies given by Hood et al (1981). The Apollo 16 subsatellite orbit gradually decayed and came close to the surface and Hood et al (1979) was able to determine magnetic anomalies on the front side of the Moon near the present equator. From these directions of magnetization I plotted the poles on a stereographic polar projection in which the center of the projection is the present lunar axis and the 0° meridian is the prime meridian facing the earth (Fig 2). When this set of pole positions was examined closely I concluded that there are two bipolar groups as shown in Fig. 2 a, b. The double stars with the 95% circles of confidence are means calculated by slightly different methods. The north magnetic poles on the present northern hemisphere of the Moon are represented by circles and on the southern hemisphere by black
The Primaeval Lunar Magnetic Field
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dots. Despite the small number of pole positions it is clear that they fall at 180° apart and define two axes very different from that found from the anomalies on the far side of the Moon. I identify the anomalies of Fig. 2a as Lower Nectarian and I have previously concluded that these rock strata were magnetized about 4 G yr ago. Fig. 2b is a polar stereographic projection showing the youngest group, which I identify as Upper Nectarian-Lower Imbrian, defining an axis distinct from the other two. I have previously given a date of .3.85 G yr ago based on Wilhelms (1985). The date of the Imbrium impact may be younger (Sttadermann et al 1991). Thus the conclusion is reached that the Moon reorientated relative to its axis of rotation in space a few times in its early history. Comparing these stereograms with those in terrestrial paleomagnetic work one is struck by the relatively few points. However it must be kept in mind that each of these poles is derived from a magnetic anomaly which averages out the magnetic directions over a very extensive area of rock and therefore I think that these results are very significant. Fig. 3 shows the paleoequators corresponding to these pole positions for the three different ages, Pre Nectarian, 4.2 (4.0) G yr, lower Nectarian age about 4 (3.9) G yr and the upper Nectarian-lower Imbrian age about 3.85 (3.85) G yr, the possible younger ages being given in brackets. The procedure for determining a “polar wandering” path for the Moon is opposite to that for the Earth. In a continent, or tectonic province, rock samples are collected of known age in strata that have not experienced relative displacement, or where correction for such movements can be made. The pole positions, calculated from the directions of remanent magnetization, are found to be grouped, apart from a scatter due to experimental errors and the geomagnetic secular variation. For the Moon the poles, calculated from palaeomagnetic directions, are found by inspection to fall into these 3 groups, the scatter in each group being consistent with what in terrestrial rocks are found to be normal in rocks of the same geological age. The problem then in lunar palaeomagnetism is to determine the ages of the groups of poles. The dots in Fig. 3 are the positions of the magnetic anomalies from which pole positions have been calculated. From the study of the geological map of the Moon the anomalies have been dated. This argument is of great importance. The absence of magnetic anomalies coextensive with
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the covering of basaltic lava flows of the mare basins in the surveys by subsatellite and by the reflected electron technique, especially by comparison with the gravity anomalies over the circular mare basins “the mascons” discovered by Muller and Sjogren (1968), left many lunar scientists skeptical of the inference of a global magnetic field. However it was pointed out Runcorn (1975) that a uniformly magnetized infinite layer, of constant thickness, has no magnetic field outside it and therefore the only magnetic signature to be expected from the mare lavas is one due to edge effects, which fall off rapidly with height on a scale given by the thickness of flows. Furthermore the remanent magnetization of the lunar basalts was found to be an order of magnitude less than that of breccia, resulting from the greater content of free iron particles found in the breccia. The magnetic anomalies must therefore arise from strata with thickness or intensity variations on a scale comparable with the horizontal scale of the anomalies, and underlying the lavas. The source of the anomalies is thus identified as the ejecta blankets, which do appear to have areal variations with the above parameters, especially topography, required to produce magnetic anomalies observable at the sub-satellite heights. The anomalies RG/1-3, C 1/2, E and KP 1-7 of Fig. 3 in the west part of the Moon are inferred to be the Fra Mauro formation ejecta from the Imbrium impact, B in the center of the Moon or the Cayley formation which may be ejecta from Serenatatis and the anomaly South of Crisium (K) is clearly on the ejecta blanket from Crisium. The poles of Fig. 2b are thus dated as upper Nectarian-lower Imbrian. Similarly the poles of Fig. 2a are dated as lower Nectarian as two anomalies J1/2, M 1/2) are on the tongues of ejecta from the Nectarian impact and Fill ,2 and F2 are likely to be on the ejecta from Moscoviense or Mendeleev. Obviously the dating becomes more difficult with the exceedingly ancient far side basins and the greater spread of the pole positions of Fig. 1, but VDG/1 -4 are close to Pre-Nectarian basins. The multi-ring basins which are named in Fig. 3 were all almost formed by impacts in relatively low latitudes at the time of their formation. I have taken Wilhelms’ (1987) dating of the multiring basins but divided the Nectarian group into Upper and Lower. Of course it was noticed last century that the very great circular maria, Imbrium, Serenatatis, and Crisium, do fall on a great circle and they are the latest of the multi-ring basins 3.85 billion years in
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5. K. Runcorn
age but that great circle we interpret now as the ancient equator. It also goes through the very famous basin Orientale which was first photographed in the early Apollo orbiting missions and near two other far side basins, Hertzsprung and Schr6dinger, which are regarded as of similar age. The existence of a lunar core The first evidence for a lunar iron core did not come from lunar paleomagnetism, or other Apollo discoveries, but from an attempt to explain a fact about the Moon known since about 1690: that it has a very markedly nonhydrostatic shape. Leplace first deduced that the dynamical ellipticity of the Moon is about 20 times greater than it would be if it were in hydrostatic equilibrium. The observation he used was Cassini’s laws of the Moon’s rotation, a discovery by the first director of the Paris Observatory. Laplace and later Harold Jeffreys both took it for granted that this nonhydrostatic shape was the result of an early distortion of the Moon. Laplace thought that in the cooling of the Moon inhomogeneities might cause a distortion of its figure. Jeifreys argued that, as the Moon was once much closer to the Earth, the tidal forces of the Earth would have given it a tidal bulge greater than the one appropriate to the present earth-moon distance. Both of them were tacitly assuming that the interior of the moon was a perfect elastic solid and would be capable of withstanding the consequent stresses (of about 50 bars) indefinitely. Because of the need to introduce the ideas of solid state physics into discussion of the long term mechanical behavior of the Earth’s mantle to provide a basis for explaining continental drift, I became skeptical with this explanation of Laplace and Jefferys, which would have required the material of the Moon to have had a creep rate less than icr20 sec~over its lifetime. Laboratory creep experiments by contrast detect only rates of lcr8sec1. I proposed instead (Runcorn 1962, 1967) that the nonhydrostatic bulge of the moon was maintained by solid state convection in the lunar mantle. From marginal stability theory we knew that convection tends to settle down so that the aspect ratio, the ratio of the lateral dimension of the convection cell to its depth, is about 1. Thus I concluded that the Moon must have a small core of about 500 km radius. If the Moon had no core the convection pattern
The Primarval Lunar Magnetic Field
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would be described by a first degree harmonic rather than the second degree and this would not fit the observations. Cassini’s laws of the Moon’s rotation are evidence that the moments of inertia of the Moon depart from hydrostatic values, and only a second degree harmonic density variation affect them. This conclusion that differentiation in the early Moon had resulted in iron settling to the center was incompatible with the established view of the Moon in the 1960’s, i.e. that it had a cold origin, and the argument did not attract attention. Even after the discovery, following the Apollo landings, of the early differentiation of the Moon, it was assumed to have involved only the outer part of the Moon -- the concept of the magma ocean. Now other evidence suggests that a lunar core exists, especially the greater depletion of siderophile elements in the Moon compared to the Earth. The hypothesis of a primaeval lunar satellite system Perhaps the most striking confirmation of the theory of an ancient core dynamo magnetic field is the discovery that the multi-ring basins were formed by low latitude impact (Runcorn 1983). The evidence is that the palaeoequators calculated from the mean palaeomagnetic poles for the 3 epochs lie close to the multi-ring basins of corresponding age. The statistical significance of these associations has been tested, using the Bingham frequency distribution which enables the lunar axis for the 3 epochs to be determined not only from the palaeomagnetic poles but also from the positions of the basin centers. The agreement is good as shown in Fig. 4 (Runcorn 1984). Thus, quite apart from any interpretation, a link emerges between the palaeomagnetic data and a fundamental question relating to lunar mechanics, but only if the palaeomagnetic directions are those of a dipole field aligned along the Moon’s axis of rotation. From this it has been concluded that a basin formed near the equator causes the Moon to reorientate relative to its axis of rotation. For a body with a lithosphere which can deform in response to solid state creep in the interior, this will result in the basin moving to the pole as shown in Fig. 5a. For the actual case observed, that of basins distributed around the equator, suggesting that the impacts followed within a time short compared to the time needed for reorientation, this will result eventually in the basins being distributed along a meridian, as shown in Fig. 5b. The second
(8)178
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conclusion is that the impacting bodies were not in heliocentric orbits but in orbit near the lunar equator i.e. they were satellites of the Moon in orbits decaying, presumably due to non-elastic disappative processes. The third conclusion focuses on the fact that the poles of these three epochs of satellite body bombardment form roughly a 90° spherical triangle. It follows that the Moon had 3 large satellites in decaying orbits, which at the three epochs broke up at the Roche limit resulting in a number of impacts near the equator. The multiring basins have slight asymmetries especially the “butterfly wings” of ejecta, which in some cases allow the direction of the impacting body to be determined (Wilhelms 1985). These data show (see Fig. 3) that for the two later epochs the bodies were orbiting near the equatorial plane at the time. Euler’s fundamental principle of the mechanics of rotation, i.e. that a body will reorientate to rotate about its axis of maximum inertia, is obviously basic to the above discussion. Hence I conclude that this agreement between palaeomagnetism and mechanics is very strong support for the existence in the early Moon of a core dynamo field. From the point of view of understanding magnetic field generation by the core dynamo, a case can be made that the Moon is the most interesting body in the solar system. Although we have information about reversals of the geomagnetic field and the changing pattern of its secular variation with time, the paleomagnetism of terrestrial rocks extends back only a fraction of the period for which we have data on the Moon. The Moon possesses the oldest records, apart from meteorites, we have of early magnetic fields, and therefore of early heat sources, in the solar system. While there is little suggestion of a change in the mean strength of the Earth’s magnetic field over the last billion years, changes in internal energies in the planets are much more likely to have been rapid in their early history than in their later, so that, correctly interpreted, the palaeo-intensity measurements on the Moon rocks contain extremely important information about the early heat sources in planetary bodies.
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The magnetic field of terrestrial planets The terrestrial planets do not present a simple picture. The Mariner 10 flyby showed that Mercury has a dipolar magnetic field of surface strength, 330 yat the equator inclined at a small angle (14°) to its axis. At first sight this seems consistent with the core dynamo theory: the high density of the planet, compression in its interior being so unimportant, requires the existence of an iron core of radius about 0.75 of that of the planet. However, Tozer (1985) has argued that it is unlikely that the core is still fluid and capable of acting as a dynamo: small planetary bodies lose their heat more quickly than large bodies. Perhaps Stephenson’s theory (1976) that the present field is a relic of a strong primeval field, which gave the crustal shell a remanent magnetization, should be considered more seriously. There has been much debate whether the small field of about 20 ‘y observed near the surface of Mars by the Soviet spacecraft Mars 1 and 2 is evidence of an internally generated field or whether the measurement can be explained by the piling up of the solar wind against the ionosphere and atmosphere of Mars. This will not be settled until the projected Mars orbiting missions are flown. However Cisowski and Collinson have shown that the SNC meteorites, which seem to be igneous rocks formed a billion years ago on Mars, thrown off by a later impact and recently fell to the Earth after orbiting the Sun, possess remanent magnetization. They conclude tentatively that this was acquired in a strong field of the order of 0.1 G and is possible evidence for an early Martian magnetic field of internal origin. That Venus has no magnetic field (less than 5 y) was surprising, for although we have no evidence concerning its internal constitution its radius and density are sufficiently close to those of the Earth to make it certain that it has a fluid iron core. Its slow rotation has been given as an explanation but, while it is uncertain which is the correct non-dimensional number, involving angular rotational velocity, that determines whether dynamo action is possible, the Coriolis force is a dominant term in the fluid cores of the terrestrial planets and the Moon. There is still doubt which source of the energy drives the Earth’s dynamo; precession, radioactive heat sources or gravitational potential energy due to separation of the inner body. One concludes that these energies are insufficient to drive the dynamo in Venus now.
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The conclusion therefore which emerges concerning the terrestrial planets-a modest one-is the critical importance of energy sources in the dynamo process. The magnetic fields of the major planets Turning to the major planets one is again struck by the diversity of their fields. The Pioneer 10 and 11 and Voyager 1 and 2 flybys give determinations of the parameters of the field of Jupiter in good agreement with the early models of the field derived from the study of the decimeter and dekameter radio noise. The Galileo orbiting mission and continuing radio astronomical studies are of great importance in testing whether, like the Earth, Jovian field has a secular variation. The small angle 9.6° of the inclination of Jupiter’s dipole to its rotation and the zero angle in the case of Saturn, are like the offset of the Jovian dipole of 0.1 of its radius and the presence in Saturn’s field of some non axial harmonic component implied by its radio noise emission, are all compatible with the core dynamo theory. Quantum theory predicted that molecular hydrogen would transform into a metallic phase at pressures of. about 106 atmospheres: this has proved fundamental to understanding the structures of the major planets. The metallic hydrogen cores of Jupiter and Saturn have radii of about 0.8 and 0.5 of their planets respectively: with a central core of Earth-like composition. The determination of the phase diagram of hydrogen under planetary pressures and temperatures show that metallic hydrogen would be fluid and conditions for dynamo action seem likely. The discovery that the dipole of Uranus was inclined to its axis at such a great angle (59°) excited speculation that the magnetic field had been caught in the process of reversal. It has only recently been shown as a result of the work of Laj et al (1991) that the geomagnetic field reverses, at least on many occasions in Cenozoic times, by the rotation of the dipole: it would therefore have been particularly exciting if this had been a likely interpretation of the Uranus field. However the crowning discovery of the Voyager mission that Neptune’s dipole had a similar large inclination (47°)
The Primaeval Lunar Magnetic Field
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and that the offset for Neptune and Uranus was 0.55 and 0.31 of their radii respectively made this interpretation untenable. The curious fact that all the major planets have magnetic dipoles antiparallel (with respect to their rotations) compared with the Earth and Mercury seems to be accidental. The magnetic fields of Uranus and Neptune alike in their large inclinations and very large quadrupole (or octupole) components single them out from all the other planets and the question arises as to what explains this. Dynamo theory has largely been concerned with spherical geometry but, in spite of its mathematical achievements, has not been able to predict the likely parameters of the fields produced. The internal structure of Uranus and Neptune is radically different from that of Jupiter and Saturn in that the internal pressures do not result in metallic hydrogen core (Hubbard 1991). Instead around Earth-like cores which seem to play no part, a spherical shell of ammoniawater should, through its electrolytic conductivity, be a likely location for dynamo action. Such a geometry, bearing in mind that the conductivity might lower the magnetic Reynolds number to a value nearer the critical one for dynamo action than in any of the other planetary metallic cores producing magnetic fields, might possibly be the basis for understanding these remarkable fields. If the Earth-like cores are supplying heat to this shell, unequally in different areas, and because they are convecting, it is not unlikely that dynamo action occurs only in (or more strongly in) one part of the shell. The dipole offset is then, not just a mathematical model of the quadrupole component of the field, but is the position of the dynamo. In a spherical shell it would be the component of the planet’s rotation perpendicular to the shell, which would provide the helicity of fluid flow thought essential to dynamo action. The offset dipole inclination directions of Neptune and Uranus appear to be very roughly perpendicular to the ammonia-water shell, in which dynamo action is taking place. The spacecraft magnetometer measurements by NASA and also by the Soviet Union have greatly expanded our knowledge of the magnetic fields of the other planets. However I think that it has to be admitted that the understanding of the core dynamo is not greatly advanced by knowing the present configuration of the fields of the planets without having evidence of their variation with time. The
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knowledge gained in the last few decades on the magnetic fields of the planets is of immense intrinsic interest but its message is that most varied fields can be generated. If it were a property of the dynamo process that fields of the most diverse parameters can be produóed, the observations obtained over the last decades could hardly be improved upon.
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REFERENCES
Baldwin, R.B. The face of the Moon. Univ. of Chicago Press 289 p (1949) Baldwin, R.B. The measure of the Moon. Univ. of Chicago Press 488 p (1963) Blackett, P.M.S. Nature 159, 658, (1947) Bullard, E.C., Proc. Roy. Soc. A. 197, 413 (1949) Chapman, S. & Bartels, J. Geomagnetism 2 vols (1940)
Chapman, S. Nature j..~Q,395 (1947) Cisowski, S., Collinson, D.W., Runcorn, S.K., Stephenson, A. and Fuller, M.D. J.geophys. Res. II A 691 -704 (1983) Elsasser, W.M. Phys. Rev. 70, 106, 1946 ~,
Elsasser, W.M. Phys. Rev. 70, 1, 1946 Elsasser, W.M. Phys. Rev. 72, 821, 1947 Coleman, P.J. & Russell, C.T. Phil. Trans. Roy. Soc. A
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Collinson, D.W., Stephenson, A. & Runcorn, S.K. Phil. Trans. Roy. Soc. A 285, 241247 (1977) Hood, L.L., Russel, C.T. & Coleman, P.S. Geophys. Res. Lett. ~ 305-308 (1978) Hood, L.L., Coleman, P.1., Russell, C.T. & Wilhelms, 0. E. Phys. Earth Planet. Int. ~Q, 291-311 (1979) Hood, L.L. Proc 12th lunar planet. Sci. conf. 1~81 7-830 (1981) Hubbard, W.B.- Nellis, W.I, Mitchel, A.C., Holmes, N.C., Limaye, S. and McCandless, P.C. Science ~ 648-651, (1991) Laj, C., Mazard, A., Weeks, R., Fuller, M., Herrero-Bervera, E. Nature 351,447 (1991) Muller, P.M. & Sjogren W.L. Science 1~1,680-684 (1968) Runcorn, S.K. Nature 195, 1150-1152 (1962) Runcorn, S.K. Proc. Roy. Soc. A
~
507-516 (1977)
Runcorn, S.K., Collinson, D.W., 0 Reilly W., Battey, M.H., Stephenson A., Jones, S.M., Manson, A.S., & Readman, P.W. Proc. Apollo II lunar sci. conf. Runcorn, S.K. Phys. Earth Planet. Int., 10, 327-335 (1975)
~,
2369-2387 (1970)
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Runcorn, S.K. Nature, 589-596 (1983) Runcorn, S.K. Phil. Trans. Roy. Soc. A ~
77-83 (1984)
Stadermann, F.S., Heusser, E., Jessburger, E.K., Lingner, S. & Stoffler, 0. Gerrochim et cosmochim Acta 55, 2339-2349 (1991) Stephenson, A., Runcorn, S.K., & Collinson, D.W. Proc. 6th lunar sci. conf. 3, 30493062 (1975) Stephenson, A. Earth Planet. Sci. Lett.
~,
454-458 (1976)
Tozer, D.C. Geophys. Surveys Z. 213-246 (1985) Urey, H.C. The planets their origin and development New Haven: Yale Univ. press 245 p (1952) Wilhelms, D.E. The geologic history of the Moon USGS Prof. paper 1348 (1981)
0273—1177/92 $15.00
Copyright @
1992 COSPAR
THE PRIMAEVAL LUNAR MAGNETIC FIELD: ITS RELEVANCE TO THE INTERPRETATION OF PLANETARY FIELDS S. K. Runcom Physics Department, Imperial College, London, U.K. (and University ofAlaska, Fairbanks, AL, U.S.A.)
ABSTRACT: The Moon today has no general magnetic field but the remanant magnetization of rocks returned and crustal magnetic fields show that it had one once. Palaeointensity laboratory measurements show the field approached 0.1 mT (1G) 3.9 Ga ago, decreasing exponentially to about 0.002 mT 3.2 Ga ago: evidence for a small iron core dynamo. Palaeomagnetic poles, determined from satellite magnetic surveys, show the Moon to have reorientated several times with respect to its axis of rotation and to have had a primaeval satellite system, the decay of their orbits producing the multi-ring impact basins. I conclude that planetary field strengths vary with evolutionary factors, such as the energy sources for convection; thus doubt is thrown on “scaling laws”. Vigorous dynamos do shut down as the Moon shows. Has the same occurred in Mars and Venus? Planetary dynamos are possible on small length scales: thus the off center dipoles of Uranus and Neptune may be local dynamos in their water-ammonia fluid spherical shells. While the mean geomagnetic and lunar fields are central dipoles, explained by the dominance of the Coriolis force in their cores, perhaps in this case the local dipoles align along the angular rotation component perpendicular to the shell. Are the Uranus and Neptune dipole inclinations so explained? Introduction Any account of what we know about the internal magnetic fields of the planets, especially in relation to the dynamo theory of their (8)165
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generation, needs to include the ancient magnetic field of the Moon. This has been one of the most surprising discoveries from the Apollo rocks. It is worth remembering that only thirty years ago we knew nothing about the magnetic fields of any of the planets other than the Earth, and in 1946 there was no theory of the origin of the Earth’s magnetic field. The dynamo theory was first proposed by Elsasser (1946 and 1947), but Blackett (1947) speculated immediately following the first discovery of a magnetic field of a star (78 Virginis) that perhaps magnetic fields of cosmic bodies might be a new property of rotating matter. At the time Einstein was much concerned with the possibility of a unified theory of electromagnetism and gravitation: so Blackett’s proposal was taken seriously as a possible observational clue. Chapman, who had written little about this question in his treatise (Chapman and Bartels 1940), was looked to for an opinion. He proposed that the theory’s prediction, that the Moon’s surface field was 100 ‘y, should be tested (Chapman 1947). He pointed out that the development of rockets and of airborne magnetometers and the use of radio for communication of information had taken vast strides during World War II and had made such an experiment possible. So he proposed the experiment, which seemed to everyone at the time science fiction, that was done by the Soviet Union two years after Sputnik finding, to an accuracy of about 100 ‘y, that the Moon had no magnetic field. The development of studies of the Moon, in the years around 1960, focused on the origin of the craters. As a result of the work of Baldwin (1949,1963) and Urey (1952) it was established that almost all the surface features of the Moon were of external origin, particularly that the craters were the result of meteorite impacts. It therefore became the established orthodoxy that the Moon was a sample of undifferentiated solar system material resulting from early cold accretion from the nebula of gas and dust. This was urged as a very strong argument for going to the Moon. On this hypothesis an iron core would not have formed and, as the core dynamo theory of Elsasser and Bullard was providing a satisfactory explanation of the main geomagnetic field, the negative result of the Soviet spacecraft experiment, showing that the Moon had no magnetic field, fitted perfectly: a liquid iron core was necessary for generating a planetary field. Consequently it was one of the most unexpected discoveries of the Apollo project that the lunar rocks
The Primaeval Lunar Magnetic Field
possessed remanent magnetization. The development of research on the palaeomagnetism of terrestrial rocks had provided insight into the various ways in which this magnetization is acquired. As a result of experiments done in a number of laboratories it became clear that the remanent magnetization of the Apollo rocks had not been picked up adventitiously since collection but had been acquired in some early process on the Moon; either at the formation of the rock or during its later exposure to meteorite bombardment or to the solar wind. The question at issue was the origin of the magnetic field in which the lavas in the mare basins and the breccia or fragments of highland rock had cooled and acquired magnetization. Because of the general opinion that the moon was a cold undifferentiated body it seemed to most lunar scientists that the magnetic field, which had magnetized the Apollo rocks, could not be an early general lunar magnetic field but must have been generated in a way unfamiliar to us on Earth. The lunar core dynamo However for reasons that I will explain later, we put forward the hypothesis that the Moon had a small iron core which could have been molten in its very early history: therefore a dynamo process was a reasonable hypothesis (Runcorn et al 1970). The important conclusion followed that the remanent magnetization of the lunar rocks was a record of the changes in this ancient lunar magnetic field. A serious obstacle to these studies was that the Apollo rocks were not oriented because they were pieces of the underlying strata broken off by impact and lying on the regolith. It is possible that methods could be developed for finding the ancient horizontal in the lava samples by magnetic and mineralogical anisotropy determinations, assuming that the flow aligns some of the grains. However at the present time there is no means for establishing the direction of the ancient magnetic field from the returned Apollo rocks. Thus the laboratories concerned with this work turned their attention to paleointensity determinations i.e. the strength of the ambient field responsible for its magnetization. There are three different methods. The Thellier-Thellier method using thermo-
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remanent magnetization (T.R.M. ), that based on anhysteritic remanent magnetization (A.R.M.) and the method that compares the natural remanent magnetization (N.R.M.) intensity with that produced in a very strong magnetic field, called isothermal remanent magnetization (l.R.M.). What these methods do is to compare the N.R.M. with what the rocks can pick up from a small laboratory field: in the case of the Thellier-Thellier method the N.R.M. is compared with that produced by cooling the rock in the laboratory from above the Curie point of its magnetic constituent which are small grains of iron. Stephenson et al (1975) discussed the change in the strength of the ambient field responsible for the magnetization of Apollo samples of different age. They found that the very earliest samples of 3.9 G yr had been magnetized by a field of almost 1 gauss (1 0~tesla) rather larger than the Earth’s polar field today. This was a result that seemed unlikely particularly to those who thought that the strength of a planetary magnetic field was related to the size of the core and to the rotation rate. However I think that it is more reasonable that the strength of the field produced by a dynamo process is dependent on the amount of energy driving the convection: even a small core can produce a high field if it is driven vigorously enough. The exponential decay of the field found by Collinson et al (1977) can be naturally interpreted as being due to the gradual diminution of the heat sources available for driving convection in the lunar core. These experiments are not easy and more results are required. The present state of knowledge has been improved by the approximate palaeointensity estimates by Cisowski et al (1983) by means of standardization of the N.R.M. which, though inherently scattered, support the finding that the field was very strong 3.9 G yr ago and declined exponentially to about .02 of this value 3.2 G yr ago. It is significant that the magnetic field was present during the period of extensive volcanism, that flooded the basins; heat was being transported in the mantle to the surface favoring core convection and dynamo action. Volcanism and field generation ceased somewhat later. Some evidence has emerged, as yet only suggestive, that the field rose prior to the maximum: if this is correct it might be a most important key to primeval lunar processes. The main conclusion of the paleointensity study is that the field in which the rocks cooled was a function of age. This is more easily explained by the dynamo process than by the various
The Primaeval Lunar Magnetic Field
other alternative suggestions as to the magnetizating agent that have been made i.e. transient magnetic fields locally generated, particularly as a result of meteorite impacts. Palaeodirections and palaeopoles Although the returned Apollo samples do not enable the direction of the ancient lunar field to be found, it has been possible to obtain palaeomagnetic directions from the magnetic anomalies observed by the 3-component magnetometer on the Apollo 15 and 16 subsatellites (Coleman and Russell 1977). It was on the far side of the moon that the magnetic anomalies were first charted: anomalies in the vertical and horizontal field intensities which arise from strata at the lunar surface. The coverage of the magnetic anomalies will remain small until the lunar polar orbiter mission is carried out. However I will show that the present small coverage is not as big a handicap as might be supposed. Coleman and Russell first fitted these very small anomalies by dipoles near the surface of the moon and they found that, to obtain an optimum fit, the dipole had to be placed about 50 km below the surface. At this depth the temperatures are above the Curie point so that rocks could not be magnetized. The correct interpretation of this important result depends on an interesting theorem: a uniformally magnetized thin disc (on the surface of the moon) produces a field exactly that of a dipole placed below the surface at the lowest point of a sphere centered at the height of the observing satellite and circumscribing the disc. So it was inferred that what was being measured by the subsatellite magnetic survey is a mean magnetization of very extensive strata, hundreds of km in lateral extent, on the lunar surface. Thus Coleman and Russell (1977), Hood et al (1978, 1979) and Hood (1981) determined directions of magnetization of different rock strata on the surface of the moon. For a core dynamo operating in the early moon the Coriolis force would have been dominant in its magnetohydrodynamic equation, as in the Earth’s core. Therefore there is a strong reason to suppose that the mean lunar magnetic field must have been a dipole aligned along the axis of rotation. For the Earth the paleomagnetism of the late Tertiary has shown that, averaging out the secular variation of the Earth’s field, an axial dipole field is found. So it is possible from lunar paleomagnetic
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data to calculate the ancient pole positions. Fig. 1 is a stereographic projection of the east and west hemispheres of the Moon with the north magnetic poles, calculated from the magnetic anomalies on the far side of the Moon, plotted. The stars in these diagrams are vector means of the two opposed groups. They are about 180°apart and lie near to the present equator of the moon. This antipodal arrangement of north poles suggests that the lunar core dynamo reversed just as the Earth’s field reverses, and that the two preferred fields are on average 180°apart as in the case of the Earth. Thus the stars define the magnetic axis of the moon and also its axis of rotation at that time. From the preponderance of Pre-Nectarian formations in this central far side of the Moon, I conclude that these pole positions are Pre-Nectarian. I have hitherto estimated this time at 4.2 G yr. I conclude that at this epoch the moon was rotating about an axis 90°to its present axis of rotation relative to its surface, although fixed in space apart from precession. As the paleoequators are at right angles to the present equator, the multi-ring basins of that age, Pre Nectarian, were formed by great impacts in low latitude as seen in Fig. 3. The absence of these very ancient impact basins on the present front side of the moon is due to the very extensive flooding of lava between 3.9 and 3.2 G yr ago covering the very faint traces of these very ancient impacts identifiable on the far side. The numbered magnetic poles were derived from anomalies modelled by Hood et al (1978): the lettered poles were later calculated from paleodirections from far side anomalies given by Hood et al (1981). The Apollo 16 subsatellite orbit gradually decayed and came close to the surface and Hood et al (1979) was able to determine magnetic anomalies on the front side of the Moon near the present equator. From these directions of magnetization I plotted the poles on a stereographic polar projection in which the center of the projection is the present lunar axis and the 0° meridian is the prime meridian facing the earth (Fig 2). When this set of pole positions was examined closely I concluded that there are two bipolar groups as shown in Fig. 2 a, b. The double stars with the 95% circles of confidence are means calculated by slightly different methods. The north magnetic poles on the present northern hemisphere of the Moon are represented by circles and on the southern hemisphere by black
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dots. Despite the small number of pole positions it is clear that they fall at 180° apart and define two axes very different from that found from the anomalies on the far side of the Moon. I identify the anomalies of Fig. 2a as Lower Nectarian and I have previously concluded that these rock strata were magnetized about 4 G yr ago. Fig. 2b is a polar stereographic projection showing the youngest group, which I identify as Upper Nectarian-Lower Imbrian, defining an axis distinct from the other two. I have previously given a date of .3.85 G yr ago based on Wilhelms (1985). The date of the Imbrium impact may be younger (Sttadermann et al 1991). Thus the conclusion is reached that the Moon reorientated relative to its axis of rotation in space a few times in its early history. Comparing these stereograms with those in terrestrial paleomagnetic work one is struck by the relatively few points. However it must be kept in mind that each of these poles is derived from a magnetic anomaly which averages out the magnetic directions over a very extensive area of rock and therefore I think that these results are very significant. Fig. 3 shows the paleoequators corresponding to these pole positions for the three different ages, Pre Nectarian, 4.2 (4.0) G yr, lower Nectarian age about 4 (3.9) G yr and the upper Nectarian-lower Imbrian age about 3.85 (3.85) G yr, the possible younger ages being given in brackets. The procedure for determining a “polar wandering” path for the Moon is opposite to that for the Earth. In a continent, or tectonic province, rock samples are collected of known age in strata that have not experienced relative displacement, or where correction for such movements can be made. The pole positions, calculated from the directions of remanent magnetization, are found to be grouped, apart from a scatter due to experimental errors and the geomagnetic secular variation. For the Moon the poles, calculated from palaeomagnetic directions, are found by inspection to fall into these 3 groups, the scatter in each group being consistent with what in terrestrial rocks are found to be normal in rocks of the same geological age. The problem then in lunar palaeomagnetism is to determine the ages of the groups of poles. The dots in Fig. 3 are the positions of the magnetic anomalies from which pole positions have been calculated. From the study of the geological map of the Moon the anomalies have been dated. This argument is of great importance. The absence of magnetic anomalies coextensive with
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the covering of basaltic lava flows of the mare basins in the surveys by subsatellite and by the reflected electron technique, especially by comparison with the gravity anomalies over the circular mare basins “the mascons” discovered by Muller and Sjogren (1968), left many lunar scientists skeptical of the inference of a global magnetic field. However it was pointed out Runcorn (1975) that a uniformly magnetized infinite layer, of constant thickness, has no magnetic field outside it and therefore the only magnetic signature to be expected from the mare lavas is one due to edge effects, which fall off rapidly with height on a scale given by the thickness of flows. Furthermore the remanent magnetization of the lunar basalts was found to be an order of magnitude less than that of breccia, resulting from the greater content of free iron particles found in the breccia. The magnetic anomalies must therefore arise from strata with thickness or intensity variations on a scale comparable with the horizontal scale of the anomalies, and underlying the lavas. The source of the anomalies is thus identified as the ejecta blankets, which do appear to have areal variations with the above parameters, especially topography, required to produce magnetic anomalies observable at the sub-satellite heights. The anomalies RG/1-3, C 1/2, E and KP 1-7 of Fig. 3 in the west part of the Moon are inferred to be the Fra Mauro formation ejecta from the Imbrium impact, B in the center of the Moon or the Cayley formation which may be ejecta from Serenatatis and the anomaly South of Crisium (K) is clearly on the ejecta blanket from Crisium. The poles of Fig. 2b are thus dated as upper Nectarian-lower Imbrian. Similarly the poles of Fig. 2a are dated as lower Nectarian as two anomalies J1/2, M 1/2) are on the tongues of ejecta from the Nectarian impact and Fill ,2 and F2 are likely to be on the ejecta from Moscoviense or Mendeleev. Obviously the dating becomes more difficult with the exceedingly ancient far side basins and the greater spread of the pole positions of Fig. 1, but VDG/1 -4 are close to Pre-Nectarian basins. The multi-ring basins which are named in Fig. 3 were all almost formed by impacts in relatively low latitudes at the time of their formation. I have taken Wilhelms’ (1987) dating of the multiring basins but divided the Nectarian group into Upper and Lower. Of course it was noticed last century that the very great circular maria, Imbrium, Serenatatis, and Crisium, do fall on a great circle and they are the latest of the multi-ring basins 3.85 billion years in
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age but that great circle we interpret now as the ancient equator. It also goes through the very famous basin Orientale which was first photographed in the early Apollo orbiting missions and near two other far side basins, Hertzsprung and Schr6dinger, which are regarded as of similar age. The existence of a lunar core The first evidence for a lunar iron core did not come from lunar paleomagnetism, or other Apollo discoveries, but from an attempt to explain a fact about the Moon known since about 1690: that it has a very markedly nonhydrostatic shape. Leplace first deduced that the dynamical ellipticity of the Moon is about 20 times greater than it would be if it were in hydrostatic equilibrium. The observation he used was Cassini’s laws of the Moon’s rotation, a discovery by the first director of the Paris Observatory. Laplace and later Harold Jeffreys both took it for granted that this nonhydrostatic shape was the result of an early distortion of the Moon. Laplace thought that in the cooling of the Moon inhomogeneities might cause a distortion of its figure. Jeifreys argued that, as the Moon was once much closer to the Earth, the tidal forces of the Earth would have given it a tidal bulge greater than the one appropriate to the present earth-moon distance. Both of them were tacitly assuming that the interior of the moon was a perfect elastic solid and would be capable of withstanding the consequent stresses (of about 50 bars) indefinitely. Because of the need to introduce the ideas of solid state physics into discussion of the long term mechanical behavior of the Earth’s mantle to provide a basis for explaining continental drift, I became skeptical with this explanation of Laplace and Jefferys, which would have required the material of the Moon to have had a creep rate less than icr20 sec~over its lifetime. Laboratory creep experiments by contrast detect only rates of lcr8sec1. I proposed instead (Runcorn 1962, 1967) that the nonhydrostatic bulge of the moon was maintained by solid state convection in the lunar mantle. From marginal stability theory we knew that convection tends to settle down so that the aspect ratio, the ratio of the lateral dimension of the convection cell to its depth, is about 1. Thus I concluded that the Moon must have a small core of about 500 km radius. If the Moon had no core the convection pattern
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would be described by a first degree harmonic rather than the second degree and this would not fit the observations. Cassini’s laws of the Moon’s rotation are evidence that the moments of inertia of the Moon depart from hydrostatic values, and only a second degree harmonic density variation affect them. This conclusion that differentiation in the early Moon had resulted in iron settling to the center was incompatible with the established view of the Moon in the 1960’s, i.e. that it had a cold origin, and the argument did not attract attention. Even after the discovery, following the Apollo landings, of the early differentiation of the Moon, it was assumed to have involved only the outer part of the Moon -- the concept of the magma ocean. Now other evidence suggests that a lunar core exists, especially the greater depletion of siderophile elements in the Moon compared to the Earth. The hypothesis of a primaeval lunar satellite system Perhaps the most striking confirmation of the theory of an ancient core dynamo magnetic field is the discovery that the multi-ring basins were formed by low latitude impact (Runcorn 1983). The evidence is that the palaeoequators calculated from the mean palaeomagnetic poles for the 3 epochs lie close to the multi-ring basins of corresponding age. The statistical significance of these associations has been tested, using the Bingham frequency distribution which enables the lunar axis for the 3 epochs to be determined not only from the palaeomagnetic poles but also from the positions of the basin centers. The agreement is good as shown in Fig. 4 (Runcorn 1984). Thus, quite apart from any interpretation, a link emerges between the palaeomagnetic data and a fundamental question relating to lunar mechanics, but only if the palaeomagnetic directions are those of a dipole field aligned along the Moon’s axis of rotation. From this it has been concluded that a basin formed near the equator causes the Moon to reorientate relative to its axis of rotation. For a body with a lithosphere which can deform in response to solid state creep in the interior, this will result in the basin moving to the pole as shown in Fig. 5a. For the actual case observed, that of basins distributed around the equator, suggesting that the impacts followed within a time short compared to the time needed for reorientation, this will result eventually in the basins being distributed along a meridian, as shown in Fig. 5b. The second
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conclusion is that the impacting bodies were not in heliocentric orbits but in orbit near the lunar equator i.e. they were satellites of the Moon in orbits decaying, presumably due to non-elastic disappative processes. The third conclusion focuses on the fact that the poles of these three epochs of satellite body bombardment form roughly a 90° spherical triangle. It follows that the Moon had 3 large satellites in decaying orbits, which at the three epochs broke up at the Roche limit resulting in a number of impacts near the equator. The multiring basins have slight asymmetries especially the “butterfly wings” of ejecta, which in some cases allow the direction of the impacting body to be determined (Wilhelms 1985). These data show (see Fig. 3) that for the two later epochs the bodies were orbiting near the equatorial plane at the time. Euler’s fundamental principle of the mechanics of rotation, i.e. that a body will reorientate to rotate about its axis of maximum inertia, is obviously basic to the above discussion. Hence I conclude that this agreement between palaeomagnetism and mechanics is very strong support for the existence in the early Moon of a core dynamo field. From the point of view of understanding magnetic field generation by the core dynamo, a case can be made that the Moon is the most interesting body in the solar system. Although we have information about reversals of the geomagnetic field and the changing pattern of its secular variation with time, the paleomagnetism of terrestrial rocks extends back only a fraction of the period for which we have data on the Moon. The Moon possesses the oldest records, apart from meteorites, we have of early magnetic fields, and therefore of early heat sources, in the solar system. While there is little suggestion of a change in the mean strength of the Earth’s magnetic field over the last billion years, changes in internal energies in the planets are much more likely to have been rapid in their early history than in their later, so that, correctly interpreted, the palaeo-intensity measurements on the Moon rocks contain extremely important information about the early heat sources in planetary bodies.
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The magnetic field of terrestrial planets The terrestrial planets do not present a simple picture. The Mariner 10 flyby showed that Mercury has a dipolar magnetic field of surface strength, 330 yat the equator inclined at a small angle (14°) to its axis. At first sight this seems consistent with the core dynamo theory: the high density of the planet, compression in its interior being so unimportant, requires the existence of an iron core of radius about 0.75 of that of the planet. However, Tozer (1985) has argued that it is unlikely that the core is still fluid and capable of acting as a dynamo: small planetary bodies lose their heat more quickly than large bodies. Perhaps Stephenson’s theory (1976) that the present field is a relic of a strong primeval field, which gave the crustal shell a remanent magnetization, should be considered more seriously. There has been much debate whether the small field of about 20 ‘y observed near the surface of Mars by the Soviet spacecraft Mars 1 and 2 is evidence of an internally generated field or whether the measurement can be explained by the piling up of the solar wind against the ionosphere and atmosphere of Mars. This will not be settled until the projected Mars orbiting missions are flown. However Cisowski and Collinson have shown that the SNC meteorites, which seem to be igneous rocks formed a billion years ago on Mars, thrown off by a later impact and recently fell to the Earth after orbiting the Sun, possess remanent magnetization. They conclude tentatively that this was acquired in a strong field of the order of 0.1 G and is possible evidence for an early Martian magnetic field of internal origin. That Venus has no magnetic field (less than 5 y) was surprising, for although we have no evidence concerning its internal constitution its radius and density are sufficiently close to those of the Earth to make it certain that it has a fluid iron core. Its slow rotation has been given as an explanation but, while it is uncertain which is the correct non-dimensional number, involving angular rotational velocity, that determines whether dynamo action is possible, the Coriolis force is a dominant term in the fluid cores of the terrestrial planets and the Moon. There is still doubt which source of the energy drives the Earth’s dynamo; precession, radioactive heat sources or gravitational potential energy due to separation of the inner body. One concludes that these energies are insufficient to drive the dynamo in Venus now.
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The conclusion therefore which emerges concerning the terrestrial planets-a modest one-is the critical importance of energy sources in the dynamo process. The magnetic fields of the major planets Turning to the major planets one is again struck by the diversity of their fields. The Pioneer 10 and 11 and Voyager 1 and 2 flybys give determinations of the parameters of the field of Jupiter in good agreement with the early models of the field derived from the study of the decimeter and dekameter radio noise. The Galileo orbiting mission and continuing radio astronomical studies are of great importance in testing whether, like the Earth, Jovian field has a secular variation. The small angle 9.6° of the inclination of Jupiter’s dipole to its rotation and the zero angle in the case of Saturn, are like the offset of the Jovian dipole of 0.1 of its radius and the presence in Saturn’s field of some non axial harmonic component implied by its radio noise emission, are all compatible with the core dynamo theory. Quantum theory predicted that molecular hydrogen would transform into a metallic phase at pressures of. about 106 atmospheres: this has proved fundamental to understanding the structures of the major planets. The metallic hydrogen cores of Jupiter and Saturn have radii of about 0.8 and 0.5 of their planets respectively: with a central core of Earth-like composition. The determination of the phase diagram of hydrogen under planetary pressures and temperatures show that metallic hydrogen would be fluid and conditions for dynamo action seem likely. The discovery that the dipole of Uranus was inclined to its axis at such a great angle (59°) excited speculation that the magnetic field had been caught in the process of reversal. It has only recently been shown as a result of the work of Laj et al (1991) that the geomagnetic field reverses, at least on many occasions in Cenozoic times, by the rotation of the dipole: it would therefore have been particularly exciting if this had been a likely interpretation of the Uranus field. However the crowning discovery of the Voyager mission that Neptune’s dipole had a similar large inclination (47°)
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and that the offset for Neptune and Uranus was 0.55 and 0.31 of their radii respectively made this interpretation untenable. The curious fact that all the major planets have magnetic dipoles antiparallel (with respect to their rotations) compared with the Earth and Mercury seems to be accidental. The magnetic fields of Uranus and Neptune alike in their large inclinations and very large quadrupole (or octupole) components single them out from all the other planets and the question arises as to what explains this. Dynamo theory has largely been concerned with spherical geometry but, in spite of its mathematical achievements, has not been able to predict the likely parameters of the fields produced. The internal structure of Uranus and Neptune is radically different from that of Jupiter and Saturn in that the internal pressures do not result in metallic hydrogen core (Hubbard 1991). Instead around Earth-like cores which seem to play no part, a spherical shell of ammoniawater should, through its electrolytic conductivity, be a likely location for dynamo action. Such a geometry, bearing in mind that the conductivity might lower the magnetic Reynolds number to a value nearer the critical one for dynamo action than in any of the other planetary metallic cores producing magnetic fields, might possibly be the basis for understanding these remarkable fields. If the Earth-like cores are supplying heat to this shell, unequally in different areas, and because they are convecting, it is not unlikely that dynamo action occurs only in (or more strongly in) one part of the shell. The dipole offset is then, not just a mathematical model of the quadrupole component of the field, but is the position of the dynamo. In a spherical shell it would be the component of the planet’s rotation perpendicular to the shell, which would provide the helicity of fluid flow thought essential to dynamo action. The offset dipole inclination directions of Neptune and Uranus appear to be very roughly perpendicular to the ammonia-water shell, in which dynamo action is taking place. The spacecraft magnetometer measurements by NASA and also by the Soviet Union have greatly expanded our knowledge of the magnetic fields of the other planets. However I think that it has to be admitted that the understanding of the core dynamo is not greatly advanced by knowing the present configuration of the fields of the planets without having evidence of their variation with time. The
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knowledge gained in the last few decades on the magnetic fields of the planets is of immense intrinsic interest but its message is that most varied fields can be generated. If it were a property of the dynamo process that fields of the most diverse parameters can be produóed, the observations obtained over the last decades could hardly be improved upon.
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